Smoke material- viewer node or through rest of noodle?
Surprised this has remained unanswered...I believe it's pretty straightforward...

The viewer node is nothing but a run-of-the-mill Emission node with color: white and strength: 1. Hit the little triangle on the left side of the Viewer node to expand it and you'll see for yourself. 

As for "ruining your render"...not at all. It's perfectly valid to plug an Emission node into the Material Output Volume socket. In fact, some people utilize this as "poor-man's volumetrics". You'll get a somewhat similar effect to a light-infused volume with absorption/scatter but with nearly instant renders versus the noise-fest of the real thing.

Attached image shows examples with a simple object made up of intersecting cubes with an arbitrarily chosen texture to add some color variation. The left instance of each pair uses standard volumetrics nodes and has a point light embedded at it's center. The right instances use an Emission node (with same settings as a Viewer node) and plugged into the Volume socket. 



(That demonstrates the rendering requirement differences. Your real-world example may demonstrate the appearance similarities better.)

Oh, and FYI, Ian Hubert uses the technique in at least one of his famous Lazy Tutorials. Don't blink or you'll miss it!